(1) Field of the Invention
The present invention relates to the manufacture of integrated circuits in general, and in particular, to a method of preventing tungsten (W) coating on the back side of a wafer during chemical vapor deposition (CVD) of W by growing a thin oxide layer on the wafer back side.
(2) Description of the Related Art
Photolithographic limits in semiconductor manufacturing determine the level of integration that is possible in very large and ultra large scaled integrated circuit, VLSI and ULSI, respectively, technologies. One unwanted contribution to the so-called detrimental de-focus issue of photolithography comes from the environment in which the manufacturing processes are performed. Namely, during the multitude of processes employed in fabricating a semiconductor device on the polished surface of a wafer, the back side (unpolished side) of the wafer can be inadvertently exposed to the same processes as directed upon the front side. In many instances, such exposure is inconsequential since following process operations are tolerant or remedial of such exposure. However, in other process steps, the result of such exposure is detrimental and can prove troublesome in those subsequent processes and can ultimately limit the yield of good semiconductor devices from the wafer. If for example, a particular process step causes large particles to exist on a wafer, the depth-of-field limitations of submicron optical-lithography tools, such as of the well-known stepper, will prevent the patterning of features to the maximum resolution required.
Of the conventional deposition methods, chemical vapor deposition (CVD) is known to generate particulates that collect on the unprotected back side of wafer, especially when depositing tungsten (W) on the front side. As is well known in the industry, tungsten is usually used for metallization of the substrate, where tungsten fluoride (WF6) is reacted with hydrogen (H2) to form W on the front side of the wafer. However, when hydrogen reduction is used for the tungsten process, hydrogen fluoride (HF) vapor is formed, which inadvertently flows to regions at the back side of the wafer. There, additional WF6 reacts with silicon (Si) or polysilicon to form a nucleation layer of tungsten (W) as well as silicon fluoride (SiF4). Continued back side reaction of tungsten fluoride with hydrogen deposits tungsten and produces additional hydrogen fluoride. The HF then reacts with the native oxide which causes additional polysilicon to be exposed to tungsten fluoride (WF6).
Some of these partially coated back side materials become detached in subsequent processes and form particulates which can cause fatal defects in the evolving semiconductor devices. Also, excessive uneven buildup of adhering deposited material on the back side of the wafer can deplanarize the back side, rendering the back side ineffective as a planar datum to assure accurate processing of the front side, such as maintaining a consistent depth of focus during a photolithographic exposure operation.
It is generally known in the art of deposition of metals onto dielectrics that especially adhesion of CVD-tungsten to dielectrics pose difficult problems. It is common practice, therefore, to deposit by sputtering, a xe2x80x9cgluexe2x80x9d layer. However, while the glue is being deposited onto the front side of the wafer, clips hold the wafer""s edge, leaving xe2x80x9cclip-marksxe2x80x9d on the wafer. Thus, the wafer back side, the wafer edge, and xe2x80x9cclip-marksxe2x80x9d remain essentially as uncoated dielectric. As a result, subsequently deposited CVD-tungsten material tends to flake off from such uncoated areas in the course of further processing, thereby contaminating processing apparatus and interfering with desired processing. Manocha, et al., in U.S. Pat. No. 5,084,415 address this problem by forming an adhesive or glue layer on the dielectric, forming metal layer, forming a protective layer on a portion of the metal layer, and etching to remove metal not covered by the protective layer, including the edges, back side of the wafer. In a modified approach in U.S. Pat. No. 4,999,317, Lu, et al., propose, prior to metal deposition, first removing the dielectric from the back side, edge, and xe2x80x9cclip-markxe2x80x9d areas of the wafer, and second, depositing an adhesive or glue layer on remaining dielectric on the front side, or face, of the wafer. Removal of dielectric material is by etching in the presence of a protective layer on the face of the wafer.
It is also possible to strip the back side of the wafer of the undesirable deposited materials, which is usually a complicated process. Alternatively, it is disclosed in the present invention a method where the back side is protected from the deposition process by forming a thin oxide layer.
It is therefore an object of the present invention to provide a method of preventing the forming of a de-focusing step on the back side of a substrate or a semiconductor wafer in order to improve the photolithographic process steps in semiconductor manufacturing.
It is another object of this invention to provide a method of forming an oxide layer on the back side of a substrate or a semiconductor wafer in order to prevent the forming of a de-focusing step.
These objects are accomplished by providing a substrate having a front side and a back side; forming a silicon dioxide layer on said back side; forming a polysilicon layer on said front side; forming an unwanted said polysilicon layer on said layer of silicon dioxide on said back side; forming a metal layer on said front side; forming islands of unwanted said metal layer on said polysilicon layer on said back side; forming an oxide layer on said unwanted polysilicon layer on said back side; forming a photoresist mask over said metal layer on said front side; patterning said metal layer on said front side using said photoresist mask; and removing said photoresist mask on said front side by using a photoresist stripper (PRS) without etching into said oxide layer and underlying polysilicon layer as well as the silicon dioxide layer on said back side, thus enabling the next photolithographic step of the manufacturing process without the effect of a defocusing high step on said back side of said substrate.